1. Field of the Invention
This invention relates to the field of wafer defect inspection, and in particular to electron optical structures used for wafer inspection.
2. Description of the Related Art
Defect inspection of semiconductor wafers and masks for IC manufacturing is an accepted production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective wafers for reprocessing, or to correct wafer processing parameters. The systems used for inspection typically use light optics and such systems are limited in resolution due to the wavelength of the measuring light (200-400 nm). As the feature sizes on semiconductor wafers drop below 0.18 xcexcm, the resolution requirements of the defect inspection tools become more demanding.
To overcome this resolution limit, electron beam inspection systems have been designed. Electron beam systems can have much higher resolution than optical systems because the wavelength of the electrons can be in the Angstrom regime. Electron beam systems are limited in the speed at which they can inspect wafersxe2x80x94in present systems, throughputs of approximately 30 minutes per square cm have been reported (Hendricks et al, SPIE vol. 2439, pg. 174). Thus to inspect an entire 300 mm diameter silicon wafer, approximately 70 hours would be required. These systems can be used in a sampling mode where only a small percent of the wafer area is inspected, thereby increasing throughput substantially. These systems have been effective for research and product development, but are impractical for use in-line in a semiconductor fabrication facility.
Multi-beam electron beam inspection tools have been proposed. These tools have electron optical designs that vary from multiple electron beam columns built on a single semiconductor wafer to a number of physically separate electron beam columns clustered together. Multiple columns built on a single wafer must be xe2x80x9cperfectxe2x80x9d columns as-fabricated, since there is no facility for adjusting the physical electron optical alignment of any column; further, the electron optical components of the column are restricted to what can be fabricated on a semiconductor wafer. The cluster of physically separate electron beam columns is costly to manufacture. Clearly, there is a need for an electron optical design that provides more flexibility in the choice of electron optical components than the single wafer design, combined with the ability to independently align electron optical components for each column, and there is a need for a design with a lower manufacturing cost than the cluster of physically separate columns.
An electron optics assembly for a multi-column electron beam inspection tool comprises one or more electron optical components which are single structures for the assembly, all other electron optical components are one per column and are independently alignable to the electron optical axes of their corresponding columns. Examples of electron optical components which can be single structures are the first accelerator electrode, the final accelerator electrode, the focus electrode mounting plate and the gun mounting plate. These single structures can provide mechanical integrity to the electron optics assembly and facilitate the manufacture of the assembly.
In a preferred embodiment, an electron optics assembly for a multi-column electron beam inspection tool comprises a single accelerator structure and a single focus electrode mounting plate for all columns; the other electron optical components are one per column and are independently alignable. The accelerator structure comprises first and final accelerator electrodes with a set of accelerator plates in between; the first and final accelerator plates have an aperture for each column and the accelerator plates have a single aperture such that the electron optical axes for all columns pass through the single aperture. Independently alignable focus electrodes are attached to the focus electrode mounting plate, allowing each electrode to be aligned to the electron optical axis of its corresponding column. There is one electron gun per column, mounted on the top of the single accelerator structure. In other embodiments, the electron guns are mounted to a single gun mounting plate positioned above the accelerator structure. The electron optics assembly can further comprise independently alignable, one per column, scanning deflectors, second focus electrodes, electron detectors, field-free tubes and voltage contrast plates coupled to the focus electrode mounting plate and independently alignable, one per column, alignment deflectors coupled to the first accelerator electrode. In preferred embodiments, the electron emitters are Schottky emitters. The columns within the electron optics assembly can be arranged in an array, in a single row or in multiple rows.